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Abstract

We present a series of long-wave-infrared (LWIR) polarimetric-based thermal
images of facial profiles in which polarization-state information of the
image-forming radiance is retained and displayed. The resultant polarimetric
images show enhanced facial features, additional texture, and details that are
not present in corresponding conventional thermal imagery. It has been generally
thought that conventional thermal imagery (MidIR or LWIR) could not produce the
detailed spatial information required for reliable human identification due to
the so-called “ghosting” effect often seen in thermal imagery of
human subjects. By using polarimetric information, we are able to extract subtle
surface features of the human face, thus improving subject identification.
Polarimetric image sets considered include the conventional thermal intensity
image, S0, the two Stokes images,
S1 and S2, and a Stokes image product called the
degree-of-linear-polarization image.

Figures (3)

(a) Conventional long-wave-infrared (LWIR) thermal image (gray scale) in
which the facial features lack the detail necessary for robust human
identification. (b) The same image except the polarimetric information has
been extracted and used to generate a
“degree-of-linear-polarization” (DoLP) image as described by
Eq. (6).

Typical set of Stokes imagery recorded by a spinning achromatic retarder (SAR)
LWIR polarimetric sensor. (Top left, rotating clockwise)
S0, S1 (normalize), DoLP, and the
S2 (normalized) polarimetric images (shown with a
false color pallet) of a human subject recorded outside under clear sky
conditions in which the ambient radiant levels were considered low, i.e., less
than 2.0W/(m2srμm).

Typical ghosting effect often seen in conventional LWIR thermal imagery (left).
Also shown is the corresponding LWIR DoLP polarimetric image (right) in which
detailed facial features are more apparent.